Engineering textbooks tell a simple story about nitrification and denitrification (Figure 1, Table 1). First, the bacterial genera Nitrosomonas and Nitrobacler (or Nitrospira) carry our ammonia (N[H.sub.4.sup.+]) oxidation to nitrite (N[0.sub.2.sup.-]) and then to nitrate (N[0.sub.3.sup.-]). Hydroxylamine (N[H.sub.2]0H) is an important intermediate product between ammonia and nitrite. Then facultative or obligate anaerobic bacteria denitrify by oxidizing organic carbon with nitrate and nitrite. Nitrous oxide (N.sub.2]0) is an important intermediate product between nitrite and atmospheric nitrogen ([N.sub.2]). This model has been successful for conventional wastewater technology and design (Grady, Daigger, & Lim, 1999; Tchobanoglous, Burton, & Stensel, 2003). Recent developments in science and technology, however, reveal that the "classic model" is specific to the treatment technologies that use it, and thus can no longer be considered a general model for treatment wetlands (Kadlec & Wallace, 2008; Wallace & Knight, 2006).
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TABLE 1 Classic Nitrification-Denitrification Stoichiometry * Process Stoichiometry Ammonia oxidation N[H.sub.4.sup.+] + 1.5[O.sub.2] [right arrow] (Nitrosomonas) N[O.sub.2.sup.-] + 2[H.sup.+] + [H.sub.2]O Nitrite oxidation N[O.sub.2.sup.-] + 0.5[O.sub.2] [right arrow] (Nitrospira and N[O.sub.3.sup.-] Nitrobacter Denitrification (many 5(C[H.sub.2]O) + 4N[O.sub.3.sup.-] [right arrow] bacteria genera) [H.sub.2] C[O.sub.3] + 4HC[O.sub.3.sup.-] + 2[N.sub.2(g)] + [H.sub.2]O * Nitrification is comprised of autotrophic ammonia oxidation plus nitrite oxidation. Ammonia and nitrite are the carbon donors. Oxygen is he electron acceptor. The bicarbonate ion (HC[O.sub.3]). is used for cell growth, and is consumed by hydrogen ions in a neutralization reaction, but is not an electron donor. Denitrification is heterotrophic. Organic carbon (C[H.sub.2]O) is used for cell growth and is also the electron donor. Nitrate is the electron acceptor. In nature, nitrification and denitrification is not a simple story. Ammonia-oxidizing bacteria, classically regarded as obligate aerobes, are now known to have an alternate path of nitrogen transformation during transient periods of anoxia that results in nitrogen loss from ammonia (Schmidt et al., 2002). Recently discovered biogeochemical processes also play important roles in the nitrogen cycle: anaerobic ammonia oxidation (Anammox) (van Loos-dtrecht & Jetten, 1998), heterotrophic nitrification (Robertson & Kuenen, 1990), aerobic denitrification (Robertson & Kuenen, 1990), methanotrophic denitrification (Raghoebarsing et al., 2006), and denitrifying oxidization by nitrate or nitrite sulfide (Gevertz, Telang, Voordouw, & Jenneman, 2000), ferrous iron (Straub, Benz, Schink, & Widdel, 1996), manganese (II) (Tebo, Johnson, McCarthy, & Templeton, 2005), and hydrogen (Smith, Ceazan, & Brooks, 1994). The global mass flux of nitrogen through these microbial pathways is not known, but it is probably large for Anammox bacteria (Op den Camp et al., 2006; Zehr & Ward, 2002). These processes appear to be ubiquitous in marine, freshwater, and estuarine sediments. Because a large denitrification mass flux from these sediments exists (Galloway et al., 2004), these newly discovered microbial pathways are probably fundamental constituents of the global nitrogen cycle.
New general models are needed to address a global nitrogen pollution problem barely considered until the late 20th century. Human production of reactive nitrogen exceeds natural fixation approximately by a factor of two (Galloway & Cowling, 2002). Vast "dead zones," such as in coastal areas of the Baltic Sea, the northern Gulf of Mexico, and the northwestern shelf of the Black Sea, are consequences of coastal eutrophication caused by widespread application of nitrogen fertilizers (Rabalais, Turner, & Wiseman, 2001). Resolution of an environmental problem of this magnitude is highly complex in terms of policy, politics, science, and engineering. From a technical perspective, the classic nitrogen model is not useful to address this issue because of the large energy requirements of nitrification and organic carbon requirements for denitrification. Environmental professionals will recognize technical opportunities to improve water quality in the face of unprecedented global nitrogen loading only if they have more complete models of nitrogen microbial transformation as part of their conceptual and design "tool kit."
Engineering applications of some of these novel processes are already emerging. The Anammox process is in commercial development (Jetten et al., 2005), and it also appears to be native to certain engineered wetland systems (Dong & Sun, 2007; Sun & Austin, 2007). Heterotrophic nitrification was discovered in a wastewater treatment system (Robertson & Kuenen, 1990) and is undergoing commercial development in engineered wetland systems (Maciolek & Austin, 2006). In addition to pollution abatement, the sharply reduced energy demand for nitrogen removal through Anammox and heterotrophic nitrification (Jetten et al., 2005; Maciolek & Austin, 2006) is a significant potential advantage to society of technologies based on these processes.
Nitrogen Removal in Treatment Wetlands
Constructed wetlands employ biogeochemical processes found in natural wetland environments. Common forms of treatment wetlands include surface flow systems (essentially man-made equivalents of natural marshes) and subsurface flow systems (where the flow is designed to pass through the root zone of the wetland vegetation growing in aggregate or engineered soils).
Engineered wetlands build upon the experience of the previous generation of passive, constructed subsurface flow wetlands by several methods to oxidize ammonia, including vertical flow flood and drain systems (Behrends, Houke, Bailey, Jansen, & Brown, 2001), vertical pulsed flow system (Molle, Lienard, Boutin, Merlin, & Iwema, 2005), and use of supplemental aeration in horizontal and vertical flow systems (Wallace, Higgins, Crolla, Bachand, & Verkuijl, 2006). Although it was once thought that wetland plant roots would provide sufficient...